Optimization of VSWR of RF coaxial cable assembly
Release time：2020-12-14
Author: unknown
AbstractIn today's science and technology society, we use electronic products more and more widely. But in order to meet people's higher requirements for the use experience of electronic products, it requires us to continuously optimize all aspects of the product. As an important part of electronic products, the optimization of RF coaxial cable components is particularly important for us, and the optimization of VSWR is the most important. The purpose of this paper is to analyze the optimization scheme by discussing the characteristics of VSWR.
Key words VSWR RF coaxial cable assembly product optimization
1 Preface
With the rapid improvement of our living standards, the impact of representative electronic products on our quality of life has been highlighted. With the development of society, electronic products also need to be optimized continuously, and the RF coaxial cable components that affect the signal quality also need us to improve their performance through technical improvement. In the RF coaxial cable module, the performance of VSWR is an important standard to measure the impedance performance of the module. How to reduce the VSWR is particularly urgent.
Influence of VSWR on RF coaxial cable assembly in many aspects
RF coaxial cable assembly is a combination of RF coaxial connector and RF coaxial cable according to certain processing technology. RF coaxial cable assembly is mainly used to connect all kinds of signal receiving and transmitting equipment, modules or antennas, transmitters, to ensure accurate, low loss, efficient and high-quality signal transmission during transmission. Therefore, RF coaxial cable assembly is the key electronic component in radio equipment, and the most important electrical parameter of RF coaxial cable assembly is VSWR. For the VSWR of RF coaxial cable assembly, which aspects can affect its performance
2.1 design impact
In the aspect of design, that is, the selection and design of the whole, that is, the selection and design of the connector and cable, and whether the connection mode used in the connection of the two parts is reasonable or not, all of these will affect the VSWR. Generally, for the type of connector and cable type specified by the user, we only need to consider whether the impedance performance of the connector and cable is up to standard and reasonable, whether the connection mode is reasonable, and whether the connection mode of these two parts can match their impedance matching.
2.2 impact on production and processing
In terms of production and processing, it is easy for us to make the size deviation of the cable due to technical reasons and external factors. After this process, the differences in the size of the finished product, the roughness of its surface and the thickness of the insulating layer covered on its surface will lead to the change of the VSWR property, thus affecting the overall quality of the RF coaxial cable assembly Quality.
2.3 influence of assembly process and assembly process control
In general, cable core stripping size, shield stripping size, cable core welding quality and cable shield stripping end quality will affect the characteristic impedance of cable assembly, and ultimately affect the VSWR of cable assembly.
2.3.1 cable core stripping head size and cable shield stripping size are too long
When the stripping size of the cable core is too long, which exceeds the required size of the conductor in the connector, it means that part of the cable core will be exposed, which will change the dielectric constant of the cable and cause the impedance of the cable itself to deviate, so that the exposed core will form a certain reflection, resulting in the increase of VSWR.
When the cable shielding layer stripping size is too long, some cables will not be covered by skin. Therefore, the exposed shielding layer will also cause the sudden change of cable characteristic impedance, which causes the impedance mismatch between the cable and the connector, resulting in the increase of VSWR.
2.3.2 quality of cable core
When welding the cable core and the inner conductor of the connector, it is necessary to prevent solder accumulation. Because the inner diameter of the inner conductor will become larger due to the accumulation of solder, the impedance mutation point will be formed at the accumulation of solder, which will change the impedance value of the module, thus changing the VSWR. Especially for the high frequency band, the effect of this phenomenon is particularly obvious.
2.3.3 quality of stripped end face of cable shielding layer
When the stripping end surface is uneven, burr or redundant, it will cause poor contact of the outer conductor of the cable assembly, thus increasing the VSWR of the cable assembly.
3. Analysis of VSWR out of tolerance
When the VSWR is out of tolerance during the test, our technicians can analyze it through the following methods, and then take relevant measures to deal with the problem.
3.1 time domain analysis
In the usual test, we use the frequency domain test method of vector network analyzer to test. The frequency domain reflection measurement is the combination of all signals reflected by the discontinuity in the cable in the whole frequency range to be tested. It is extremely difficult to estimate the mismatch position with this method. However, time domain (TDR) analysis is an effective tool. Time domain measurement is the influence of each discontinuity of time (or distance) function, so it is very easy to locate faults, identify impedance changes in connectors, and determine the location and size of mismatch.
In general, S11 ratio measurement of vector network analyzer is used for time domain transformation measurement. S11 reflection measurement is not simply the size of the reflected signal received by the real receiver, but the ratio measurement result between the real receiver and the reference receiver. In addition, the measurement of S11 ratio can remove the systematic error through calibration, so the time and amplitude data are calibrated and accurate.
There are two kinds of time-domain measurement: time-domain band-pass mode and time-domain low-pass mode. Time domain low-pass mode is the simulation of traditional time domain reflectometer measurement, and provides two ways of step signal and shock signal. It provides the best resolution in a certain bandwidth in frequency domain. Low pass measurement mode can determine the type of impedance (resistance type, capacitance type, inductance type) at the discontinuity; time domain band-pass mode is used to test the impulse response characteristics of the device, which is suitable for testing any device in any frequency range. It is especially beneficial for fault location measurement. It can determine the mismatch position, but its disadvantage is that it can not point out the mismatch type. In general, we use band-pass mode to measure, so that we can quickly locate the mismatch position and size, and take relevant measures to compensate.
3.2 theoretical calculation method
Theoretical calculation, as the name suggests, is to carry out a series of calculations based on our theoretical knowledge of VSWR. This method can help us to find out whether the interface of RF coaxial connector matches and whether the dislocation compensation is appropriate, including the influence of slotting of inner conductor and outer conductor on impedance, and the influence of mechanical tolerance of connector on electrical performance parameters of products.
3.2.1 calculation method of impedance
Voltage standing wave ratio (VSWR) is a parameter that reflects the nonuniformity of cable impedance. Usually, we transform the analysis of VSWR into the analysis of cable impedance nonuniformity. "Characteristic impedance" is the most frequently mentioned index in RF cables, connectors and RF cable components. The maximum power transmission and minimum signal reflection depend on the characteristic impedance of the cable and the matching of other components in the system. If the impedances match perfectly, the loss of the cable is only the attenuation of the transmission line, and there is no reflection loss. Assuming coaxial installation, the characteristic impedance Z0 is. Where, ε is the dielectric constant of the cable insulation medium, D is equivalent to the inner diameter of the outer conductor, and D is equivalent to the outer diameter of the inner conductor. In general, the dielectric constant is fixed when the type of insulating medium is selected. Therefore, through the above formula, we know that to change the impedance value of the cable, we can reduce the inner diameter of the outer conductor or increase the outer diameter of the inner conductor.
3.2.2 calculation method of insulation medium compensation
In order to support the conductor in the connector, there are usually insulation supports in the connector. Due to the intervention of insulating medium, the size of inner and outer conductors of connector will change suddenly, which will inevitably cause discontinuous capacitance on the surface of insulation support and form reflection. Therefore, the thickness of insulation support B can cause the VSWR of RF coaxial connector.
The thickness B of insulation support is:
。 Where FC is the theoretical upper limit frequency of the air coaxial cable, f is the working frequency, λ G is the wavelength of the working frequency, and ε R is the relative dielectric constant of the insulation support. From the formula, we get that the resonant frequency of the connector increases with the decrease of the dielectric constant ε r when the insulation support thickness B is fixed. When B approaches zero, it becomes an air coaxial line, and its resonance frequency is determined by its cut-off frequency. Therefore, when the insulation support thickness B is less than the diameter D of the outer conductor of the connector, and the thinner the B, the better.
3.2.3 deviation of characteristic impedance with different axial degrees
The cross sections of the inner and outer conductors of the connector are different due to the processing and assembly process. Due to the effect of different axial degrees, the distributed capacitance in the cable is changed, resulting in the change of impedance.
4 ways to optimize VSWR
After analyzing the factors that affect VSWR, it is not difficult to summarize the methods that are helpful to optimize it
(1) First of all, it is necessary to reflect the selection of materials. For the inner and outer conductor materials, we need to choose the materials with good consistency and high quality;
(2) Strengthen the management of each link in the process of electrical connector processing, and improve the process method and test tooling;
(3) For the improvement of the assembly process: strictly control the stripping size of the cable core wire, at the same time use the relevant tooling or processing equipment to ensure that the insulation layer and shielding layer stripping end face are flush, without burr; strictly control the control of the amount of solder of the inner conductor and outer conductor.
5 Conclusion
VSWR plays a decisive role in RF coaxial cable assembly. Whether its performance is excellent or not is the key. From its performance, we can see whether it can play a good role in the products. When the quality of products used in our daily life is improved, the improvement of our quality of life is obvious. Any technology is from imperfect to perfect, we should keep the attitude of research, constantly optimize our technology, so as to improve our social living standards.
reference
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Author unit
Shaanxi Huada Technology Co., Ltd. Xi'an 710054
Reprinted from: https://www.xzbu.com/1/view-11615998.htm
